This "argument" in favor of gay marriage is common. It is also an emotional appeal masquerading as a serious argument.
It is based on the false assumption that sexual attraction is equivalent to love. If sexual attraction is love, and any attempt to forbid love is discriminatory, then homosexual marriage must be clearly acceptable.
The proponents of this argument ignore the fact that the same logic could be used to justify “consensual” incest, or polyamory, or any type of relationship between two consenting partners.
Why should the state forbid a brother and sister who are sexually attracted to one another from marrying? Because incest is disgusting? Many people say the same about gay marriage. Because the children the couple bear might suffer disease? The couple can simply contracept, and refuse to bear children.
If a 30 year old teacher and a 15 year old student both consent to a relationship, then why should the state interfere? Because the relationship is inherently coercive? Who determines whether or not a relationship is inherently coercive, anyway?
If sexual attraction is synonymous with love, any type of consensual sexual relationship is acceptable. After all, love is love!